1. Field of the Invention
This invention relates to electroconductive materials or articles, such as functionally gradient materials, in which electrical resistance varies along a continuous gradient, to a method of producing the electroconductive materials or articles, and to a motor produced by the electroconductive material.
2. Description of the Related Art
Functionally gradient material is one kind of electroconductive material. The functionally gradient material is used under severe circumstances, such as space planes, fusion reactors and so on. The functionally gradient material combines together plural kinds of material having different functions. In these materials, the composition gradient of the plural materials varies over a certain portion of an article produced.
For instance, an example of an application of a functionally gradient material developed by the assignee of the present application is disclosed by "Residual Stress Characteristics of Functionally Gradient Materials" (Journal of the Ceramic Society of Japan 100[4] 476-481 (April 1992)). The combination of materials of the functionally gradient material disclosed in the above publication comprises a ceramic and metal having a very good ability to reduce thermal stresses due to temperature differences and/or different material constants. The material system used there prevents the original composition form of functionally gradient material from changing due to diffusion under high temperature.
One method of producing the functionally gradient material having a good ability to reduce thermal stresses is a method of stacking material so as to change the composition, such as a sintering technique which comprises the steps of grading the powders, stacking, forming and sintering. Another method is the thermal spraying method, and so on.
However, in the functionally gradient material produced by the method described above, though the composition changes continuously, in a broad view, interfaces between different kinds of material and air holes exist in the functionally gradient material, in a narrow view. Consequently, it is difficult to produce an article with properties such as electrical resistance which do not change sharply due to the difference of the composition and the existence of air holes.
Further, a deep-slot squirrel-cage rotor is known for an induction motor of the type which is disclosed in Earlier Japanese Laid-open Utility Model Publications Sho. 58-103561, 58-103562, 60-119882 and 61-52478 and Earlier Japanese Laid-open Patent Publication Sho. 59-198858. These deep-slot squirrel-cage rotors comprise at least two kinds of electrical conductors. In more detail, the upper portion of the squirrel-cage rotor is produced from a high resistance electrical conductor, and the lower portion of squirrel-cage rotor is produced from a low resistance electrical conductor. The upper portion and lower portion are joined. Accordingly, the deep-slot squirrel-cage rotor has characteristics similar to a double squirrel-cage rotor. The deep-slot squirrel-cage rotor is devised to increase the heat capacity of the electrical conductor.
However, the current value and the calorific value at the upper portion are different from those at the lower portion. Consequently, since differences of thermal elongation stress are caused between the upper portion and the lower portion, it is easy for cracks to be formed in the joined interface between the upper portion and the lower portion when the induction motor is used for repeated starting at high frequency.